1. Field of the Invention
The present invention relates to methods for manufacturing surface-emitting laser devices, optical scanners, image forming apparatuses, and oxidation apparatuses; and more specifically, to a method for manufacturing a surface-emitting laser device that emits laser light in a direction perpendicular to a substrate, an optical scanner having the surface-emitting laser device manufactured by the method, an image forming apparatus having the optical scanner, and an oxidation apparatus that uses vapor to oxidize an object.
2. Description of the Related Art
A vertical cavity surface-emitting laser (VCSEL) device emits laser light in a direction perpendicular to a substrate, and has received attention due to its competitive price, low power consumption, small size, compatibility with two-dimensional devices, and high performance when compared with an end-surface-emitting semiconductor laser device that emits laser light in a direction parallel to a substrate.
A surface-emitting laser device is applied to the optical source (having an oscillation wavelength of a 780 nm band) of an optical writing system in a printer, a writing optical source (having an oscillation wavelength of a 780 nm band and an oscillation wavelength of a 850 nm band) in an optical disk device, and the optical source (having an oscillation wavelength of a 1.3 μm band and an oscillation wavelength of a 1.5 μm band) of an optical transmission system such as a LAN (Local Area Network) using optical fibers. Moreover, the surface-emitting laser device is expected to serve as a light source for transmitting laser light between substrates, inside a substrate, between the chips of a large scale integrated circuit, and inside the chip of a large scale integrated circuit.
The surface-emitting laser device has a confinement structure so as to improve a current inflow efficiency. For example, the confinement structure (hereinafter also referred to as an “oxidized confinement structure” for convenience sake) (see, for example, Patent Document 1) obtained by selectively oxidizing an Al (aluminum) As (arsenic) layer is generally used. The oxidized confinement structure is manufactured in the following manner. That is, a predetermined size of a mesa having a selectively oxidized layer made of p-AlAs exposed at its side surface is formed and then placed in a high-temperature vapor atmosphere to selectively oxidize Al from the side surface of the mesa. With the oxidation of the Al, a non-oxidized region is formed in the selective oxidized layer near the center of the mesa. The non-oxidized region is a region (current passing region) through which the driving current of the surface-emitting device passes.
The refractive index of the layer where Al is oxidized (AlxOy) in the oxidized confinement structure (hereinafter briefly described as an “oxidized layer”) is about 1.6, which is smaller than that of a semiconductor layer. Accordingly, since a refractive-index difference in a transverse direction occurs in a resonator structure and light is trapped at the center of the mesa, light-emitting efficiency can be improved. As a result, excellent characteristics such as a low threshold current and high efficiency can be realized.
Meanwhile, an oxidation rate in the selectively oxidized layer containing Al correlates with the thickness of the selectively oxidized layer and the compositions of Al and As (see, for example, Non-Patent Document 1), and is extremely susceptible to an oxidation temperature, vapor concentration, etc.
Further, due to a slight difference in the compositions and the thickness of the selectively oxidized layer, the size of the current passing region is varied among objects to be oxidized even in the same oxidation time. In addition, plural mesas are generally formed in the object to be oxidized, and the size of the current passing region is varied even with the same object to be oxidized due to the mesas. The variations in the size of the current passing region cause variations in an oscillation characteristic (such as an optical output), which in turn directly leads to the reduction of yields.
Particularly, the area of the current passing region in a single-mode device is smaller than that of the current passing region in a multi-mode device. Therefore, variations in the size of the current passing region largely affect variations in the characteristic of the device. In addition, if the area of the current passing region becomes larger than a desired area, the one which should serve as the single-mode device shows the behavior of the multi-mode device.
In view of this, a method for making uniform the oxidation amount of an object to be oxidized and appropriately controlling an oxidation amount among objects to be oxidized has been achieved (see, for example, Patent Document 2) using a semiconductor oxidation apparatus that has a heating stage for heating the object to be oxidized and a vapor supply unit for oxidizing a selectively oxidized layer in a sealed vessel.
Further, a method for performing an oxidation step on one object to be oxidized plural times to absorb a difference in oxidation rates due to the differences in the compositions and the thicknesses of selectively oxidized layers has been known (see, for example, Patent Document 3).
Further, Patent Document 4 discloses a susceptor for a vapor-phase growth apparatus that has grooves at its front surface to form the receiving portion of a semiconductor substrate and coats the front surface of the semiconductor substrate with a film by vapor-phase growth under high temperature. In the susceptor, the bottom surface of the receiving portion is formed into a convex shape, and a part of the bottom surface is provided with a projection for supporting the semiconductor substrate.
Further, Patent Document 5 discloses a susceptor for vapor-phase growth of a vapor-phase growth apparatus with respect to a semiconductor substrate having a crystal orientation (100), wherein the bottom surface of the spot facing of the susceptor on which the semiconductor substrate is mounted is formed into a convex sphere or a convex cone.
Meanwhile, when a thin film is being grown on a substrate by, for example, a metal organic chemical vapor deposition (MOCVD) method, the temperature (growth temperature) of the substrate is in the range of about 500 through 800° C. After the growth of the thin film, the substrate is cooled to room temperature. Here, since the thin film is grown on the substrate with excellent lattice matching, warpage corresponding to a difference in the coefficients of thermal expansion between the substrate and the thin film is caused in an object to be oxidized as shown in FIG. 1. At this time, the amount of warpage at the center of the object to be oxidized could be as large as 100 through 200 μm.
In a conventional semiconductor oxidation apparatus, inventors placed an object to be oxidized on a heating stage through a flat sample tray to oxidize a selectively oxidized layer, and found a relationship between the amount of warpage at the center of the object to be oxidized and the oxidation rate of the selectively oxidized layer. FIG. 2 is a graph showing the relationship. As shown in FIG. 2, the amount of warpage of the object to be oxidized correlates with the oxidation rate, indicating that the amount of warpage of the object to be oxidized largely affects the oxidation rate.
Further, the inventors found a relationship between a distance from the center of an object to be oxidized and the amount of warpage of the object to be oxidized having warpage of about 130 μm at its center. FIG. 3 is a graph showing the relationship. In addition, FIG. 4 is a graph showing a relationship between the distance from the center of the object to be oxidized and the oxidation rate of the selectively oxidized layer when the object to be oxidized was placed on a heating stage through a flat sample tray to oxidize the selectively oxidized layer in a conventional oxidation apparatus. In this case, the oxidation rate at the center of the object to be oxidized is slower than that at the periphery, which in turn leads to variations in the size of the current passing region of the object to be oxidized. As a result, a manufacturing yield is reduced.
Moreover, the object to be oxidized may not uniformly warp such that it would have been symmetrical about the center of the object to be oxidized in outward directions, but may warp nonuniformly in outward directions. FIG. 5 is a graph showing an example of a relationship between the distance from the center of an object to be oxidized and the amount of warpage in the 0° direction and the 90° direction. Here, a difference in the amount of warpage is confirmed between the 0° direction and the 90° direction. Note that the definitions of the 0° direction and the 90° direction are explained in FIG. 5.
When an object to be oxidized having such a nonuniform warp amount in a plane was oxidized in the process of manufacturing a surface-emitting laser device, it was confirmed that the distribution of the areas of current passing regions is different depending on the directions as shown in FIG. 6. As shown in FIG. 6, the distribution of the areas of the current passing regions is uniform in the 0° direction but nonuniform in the 90° direction. This indicates that the distribution of temperatures in the oxidation process is nonuniform in the 90° direction. The difference in the areas leads to variations in area of the current passing region of the object to be oxidized. As a result, the manufacturing yield of the surface-emitting laser device is reduced.
Thus, the inventors have obtained knowledge that the warpage of an object to be oxidized is one of the factors causing variations in the current passing region other than the composition and the thickness of a selectively oxidized layer.    Patent Document 1: U.S. Pat. No. 5,493,577    Patent Document 2: JP-A-2006-228811    Patent Document 3: JP-A-2006-303415    Patent Document 4: JP-A-62-4315    Patent Document 5: JP-A-62-262417    Non-Patent Document 1: “Advances in Selective Wet Oxidation of AlGaAs Alloys,” by Kent D. Choquette et. al., IEEE Journal of Selected Topics in Quantum Electronics, Vol. 3, No. 3, pp. 916-926, 1997